Let me make a confession before giving you my conclusion: people accuse me so much of being in Topping's pocket based on good reviews I have given them that I secretly wish that some of their products don't do well so that I can say that and get out of jail!!!
Alas, that opportunity is not here yet.
With respect, I'm going to call you on a few things in this review:
We see nice output level of 2 volts but with slight channel imbalance. Distortion is very low. SINAD which is the inverse of that, shows distortion and noise free dynamic range of 104 dB, well in excess of what we need for CD playback (93 dB with dither).
Distortion is very low, but you must be still using a 24 bit, 44.1K signal from the AP as the THD is below the theoretical lower limit for 16bit? (0.0015% or so).
Saying it is 'well in excess of what we need for CD playback' and comparing the two is a little misleading as CD is 16 bit, 44.1KHz.
Considering
if you are using a 24/44.1 or even a 16/44 signal (from the AP), the tested S/N is
pretty poor actually, and based on the Topping specs, the test unit doesn't remotely meet its spec on residual noise. Using your numbers, the residual noise comes in at around 11.8uV vs a rated spec of 3.5uV (low level settings).
Over three times worse than rated spec. Consider also your test is not even wideband, you are filtering above 22.4K (on the AP) and these Topping D/As are only going to get worse figures and considerably noisier above 44.1fS.
Basically, if I read your settings correctly, you are testing a DSD capable D/A at its lowest sampling rate, bandwidth limiting the FFT and feeding it a nice clean 24 bit signal. It should be achieving considerably better S/N than it does, after all, we had 16 bit CD players back in the late 1980s that hit 117dB S/N on
actual tests- ie, residual noise under 2.8uV. (and that is without 0 level/LSB muting)
Let's see what it actually does on a 16/44 signal, a 24/96 and 24/192 as I don't reckon anyone has files or musical content at 24/44. And if I have misinterpreted your results, I apologize.